Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere.
Evaporative leaks may be identified using engine-off natural vacuum (EONV) during conditions when a vehicle engine is not operating. In particular, a fuel system may be isolated at an engine-off event. The pressure in such a fuel system will increase if the tank is heated further as liquid fuel vaporizes. As a fuel tank cools down, a vacuum is generated therein as fuel vapors condense to liquid fuel. Vacuum generation is monitored and leaks identified based on expected vacuum development or expected rates of vacuum development. In some vehicles, such as in plug-in hybrid electric vehicles, engine run time is limited and a vacuum pump is required to perform leak detection. The vacuum pump may be included in an evaporative leak check module (ELCM) which draws vacuum across a reference orifice to obtain a reference vacuum to which evacuated fuel tank vacuum is compared.
However, both EONV and ELCM based leak tests are prone to error when a fuel with a high Reid Vapor Pressure (RVP) is present in the fuel system. For an EONV test, highly volatile fuel may produce a pressure which counteracts leaks in the fuel system, causing a false pass during the pressure-rise portion of the EONV test. For an ELCM test, the fuel vapor of a high RVP fuel may counteract the vacuum pull of the ELCM pump, causing a false failure during the ELCM test.
The inventors herein have recognized the above problems, and have developed systems and methods to at least partially address the problems. In one example, a method for an evaporative emissions leak test, comprising: adjusting a pressure threshold based on a fuel volatility of a fuel contained in a fuel tank; and performing the evaporative emissions leak test based on the adjusted pressure threshold. In this way, both positive pressure tests and negative pressure tests may compensate for fuel volatility in setting pressure thresholds. For example, vehicles using highly volatile fuel (e.g. winter fuel) during warmer ambient temperatures may set incorrect pressure thresholds based on ambient temperature, barometric pressure, etc. that may cause false pass results for positive pressure tests and may cause false fail results for negative pressure tests. By determining fuel volatility and adjusting a pressure threshold based on the fuel volatility, a more robust and accurate evaporative emissions leak test may be employed without adding additional components to a fuel system.
In another example, a method for an evaporative emissions system leak test, comprising: determining a reference vacuum threshold; venting fuel vapor from a fuel tank; determining a fuel Reid Vapor Pressure of the fuel vapor; adjusting the reference vacuum threshold based on the fuel Reid Vapor Pressure; drawing a vacuum on a fuel tank with an evaporative leak check module; and indicating degradation of the evaporative emissions system based on the adjusted reference vacuum threshold. In this way, false failures may be reduced for an evaporative leak check module based test. In a fuel system using a fuel with a high Reid Vapor Pressure, the fuel vapor may counteract the vacuum pull of an evaporative leak check module. As such, the resulting test vacuum may not reach the expected reference vacuum threshold, indicating a leak in the fuel system even if the system is intact. By compensating for the fuel Reid Vapor Pressure, an accurate reference vacuum threshold may be determined, resulting in a more accurate test with fewer false failures.
In yet another example, a method for an evaporative emissions system leak test, comprising: responsive to an engine off condition, closing a canister vent valve; determining a resulting pressure of a fuel tank; determining a Reid Vapor Pressure of a fuel in the fuel tank; determining a threshold pressure based on the fuel Reid Vapor Pressure; and comparing the resulting pressure of the fuel tank to the threshold pressure. In this way, false passes may be reduced for an engine-off natural vacuum test. In a fuel system using a fuel with a high Reid Vapor Pressure, the fuel vapor may result in an increased fuel tank pressure during the pressure rise portion of the test. This may indicate an intact fuel system, when in fact the fuel system is degraded. By compensating for the fuel Reid Vapor Pressure, an accurate pressure threshold may be determined, resulting in a more accurate test with fewer false passes.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.